Portia, a genus of small jumping spiders, has the reputation for being a highly effective hunter that uses superb intellect to take down large, poisonous spiders as prey. As reputations go, it's not a bad one. A Google search reveals the little spider to be a veritable eight-legged Einstein, but just how smart can an animal with a pinhead-sized brain possibly be? Fiona Cross from the University of Canterbury, New Zealand, and Robert Jackson from the International Centre of Insect Physiology and Ecology, Kenya, have spent years investigating the cognitive tricks that Portia extracts from that little brain.

One such trick is Portia’s ability to observe potential prey from a distance and then pre-plan a complex route from that same vantage point so as to sneak up behind the unsuspecting prey item. Portia is remarkably effective at this and has the rare ability to plot routes that move the prey out of visual contact for periods at a time. This behaviour implies that Portia creates a mental image of a predatory scene and commits it to a working memory, recalling it when the scene comes back into view. The accuracy with which Portia builds and recalls this mental image is of great interest to Cross and Jackson, and this particular study saw them investigate Portia’s ability to comprehend numbers, as the little spider regularly encounters numerous prey spiders at a time.

The duo started by building an experimental apparatus that allowed individual Portia specimens to view a modifiable prey scene, plot an attack route and then set out on their predatory journey. The investigators maintained the consistency of each Portia attack route by exploiting the spider's known aversion to water by surrounding the single route from the initial viewing platform to the prey scene with water. This route removed the prey scene from the spider's field of view for about a minute, during which time the experimenters could modify the number of prey items (lures made from preserved specimens of Portia’s favourite food, Argyrodes spiders). To interpret the spiders’ responses to modified prey scenes, the duo enlisted the expectancy-violation method of behaviour assessment, whereby violations of expected behaviours are interpreted as the test subject's comprehension that something has changed. In this case, the time Portia took to re-analyse a prey scene upon its reappearance in their field of view, and the number of Portia that followed through with an attack, were analysed for expectancy violation.

The results reveal that Portia is surprisingly competent with numbers. When the spiders re-emerged into a prey scene with an altered number of prey items, they spent more time inspecting the scene for further information, and were less likely to mount an attack, than when the prey item number was unaltered. Furthermore, Cross and Jackson observed no significant effects when the original prey number and altered prey number were both ≥3, suggesting that Portia represents 1 and 2 as discrete number categories, and anything ≥3 as the same number category, which the duo called ‘many’. The methods cannot reveal just what Portia was pondering while investigating a mismatched prey scene, but based on how Portia’s principal eyes function, the pair suspect the spiders were individuating the objects in their present field of view and comparing them with those in their working memory, a sluggish but effective process known as ‘proto-counting’.

And so Portia’s numerical competency will be added to its googleable lore, building on what is already a stellar reputation. That's a good thing because, frankly, few spiders can boast such a claim. But everyone loves a genius, even one with eight legs.

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Representation of different exact numbers of prey by a spider-eating predator
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